1. Field of the Invention
The subject invention relates to a process for the preparation of polyoxyalkylene block polyethers. More particularly, the invention relates to a process for the preparation of block polyoxyalkylene polyethers having one or more polyoxyethylene blocks and at least one block derived from a higher alkylene oxide. The use of cesium hydroxide to catalyze the oxyethylation results in polyethers having enhanced properties.
2. Description of the Related Art
Polyoxyalkylene block polyethers are well known commercial products having many uses, the most important of which is their use as nonionic surfactants. Polyoxyalkylene block polyether surfactants generally have both hydrophobic and hydrophilic blocks, and are described, for example, by Lundsted in U.S. Pat. No. 2,674,619 and by Jackson and Lundsted in U.S. Pat. Nos. 2,677,700 and 3,036,118. These references also disclose the preparation of such polyoxyalkylene block polyethers by oxypropylating an initiator molecule possessing two or more active hydrogens in the presence of a basic catalyst such as sodium or potassium hydroxide. The polyoxypropylene hydrophobe is then oxyethylated to produce external hydrophiles, or, in certain cases, the oxypropylation and oxyethylation may be reversed to produce "reverse" non-ionic surfactants having an internal hydrophile and external hydrophobes.
Diblock polyoxyalkylene polyethers or triblock polyoxyalkylene polyethers capped on one end are also useful products. These products are generally prepared by sequentially oxyalkylating a monofunctional initiator molecule such as an alkanol or phenol. To prepare diblock polyethers by this method, the initiator is first reacted with a higher alkylene oxide, that is, one having three or more carbons. The resulting hydrophobe is then oxyethylated. In certain applications the oxyalkylation may be reversed. Triblock polyethers are similarly prepared, but with a third oxyalkylation utilizing the same alkylene oxide as used for the first oxyalkylation.
For example, a triblock polyoxyalkylene polyether may be conventionally prepared, as shown in the reaction scheme below, by first oxypropylating a difunctional initiator molecule followed by oxyethylation. In these reaction schemes, --OP-- and --PO-- represent oxypropyl residues derived from propylene oxide while --OE-- and --EO-- represent analogously derived oxyethyl groups. ##STR1## An analogous monofunctional, mono-capped triblock polymer may be prepared by starting with a monol, R--OH, such as methanol, butanol, or benzylalcohol and altering the oxyalkylation sequency as follows: ##STR2## Such mono-capped block polyethers where the cap is joined to the block polyether by an ether linkage are hydrolytically stable and have been shown to possess different physical and chemical properties as compared to their non-capped analogues including modified surface activity and increased thermal stability.
The polyoxyalkylene polyethers described above have proven useful in numerous applications, particularly those requiring surface active properties such as detergents, foaming and defoaming agents, emulsifying and dispersing agents, and as thickeners in aqueous systems. However, despite their great utility, the methods of preparation previously described never results in a single, uniform product molecule, but in a cogeneric mixture containing molecules with widely varying total molecular weights as well as widely varying hydrophobe and hydrophile weights. This is particularly true as the molecular weights increase. Although it is well known that block polyether surfactants having uniform, narrow molecular weights and compositions possess properties markedly different from those of ordinary commercial products, it has been impossible to prepare such specialty products without inordinate expense.
It has now been surprisingly discovered that polyoxyalkylene block polyethers having narrow molecular weight distribution, uniform composition, and unexpectedly low levels of unsaturation may be simply and economically prepared through the use of cesium hydroxide catalysis for at least the oxyethylation portion of the polyether synthesis, and preferably for both oxyethylation and oxypropylation.
The use of cesium hydroxide as a polyoxypropylation catalyst has been proposed in U.S. Pat. No. 3,393,243. According to this reference, the use of cesium hydroxide as opposed to conventional sodium or potassium hydroxide catalysts in the synthesis of polyoxypropylene glycols prevents the elimination reaction at the polyether chain terminus, which ordinarily results in forming allylic unsaturation and, at the same time, lowers and broadens the molecular weight of the product polyoxypropylene glycols.
A mechanism for the elimination disclosed in U.S. Pat. No. 3,393,243 is discussed in Ceresa, Block and Graft Copolymerization, vol. 2, published by Wiley-Interscience at page 18. The mechanism apparently involves hydrogen abstration via a specific cyclic transition state which may be represented as follows: ##STR3## The unsaturation formed increases as a direct function of equivalent weight. Eventually a point is reached wherein further propylene oxide addition fails to increase the molecular weight.
When oxyethylation rather than oxypropylation is performed, as in the preparation of block polyethers, the use of cesium hydroxide as a catalyst has not been contemplated. The reason for this is that while it is readily conceived that polyoxypropylene glycols may react by the above mechanism, the same cannot be true for polyoxyethylene glycols or for oxyethylated polyoxypropylene glycols containing more than one oxyethyl group. Thus, until now, such block polyethers have been prepared with less expensive sodium and potassium hydroxide catalysts.
For example, when a single oxyethyl group is added to a polyoxypropylene glycol, the elimination mechanism may be written thusly: ##STR4## However, when more than one oxyethyl group is present, the requisite transition state cannot be achieved, and thus it had not been thought that the elimination products could affect in the polymerization reaction: ##STR5## Consequently, no eIimination, no unsaturation formation, and therefore no lowering of the polyether molecular weight is expected during ethylene oxide addition and, in fact, none has been detected heretofore.